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Q: All motion is relative. Does this apply to rotation? ( No Answer,   4 Comments )
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 Subject: All motion is relative. Does this apply to rotation? Category: Science > Physics Asked by: antell-ga List Price: \$10.00 Posted: 23 Jun 2002 08:53 PDT Expires: 25 Jun 2002 18:16 PDT Question ID: 31905
 ```I understand that according to Einstein's Special Theory of Relativity (which concerns bodies moving at a constant speed) it is not possible to say what the absolute speed of any body is. Only its speed relative to another body can be determined. For example, if a car is, according to the usual way to viewing matters, travelling at 20mph relative to the road, it would be equally valid to say that the car is stationary and that the road is travelling at 20 mph relative to the car. My question is, does this principle also hold good in the case of angular motion? For example, instead of saying that the earth rotates on its axis once every 24 hours (relative to the stars) would it be equally valid to say that the stars orbit the earth one every 24 hours relative to the earth? If this would not be an equally valid way of looking at it, why not? If this would be an equally valid way of looking at it, does that mean that the stars which orbit the earth are necessarily (from this frame of reference) travelling faster than the speed of light? If not why not?```
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 ```The answer given above is incorrect. While the theory of special relativity tells us that all inertial reference frames are equivalent, this equivalence does not hold for rotational reference frames. Constant rotational motion requires a constant acceleration, and acceleration is something you can measure without ambiguity, unlike velocity which is defined only with respect to a given reference frame. And no, it is never true that the relative velocity of two ships exceeds the speed of light. If two ships are on a collision course, and each of them is traveling 3/4 the speed of light relative to the fixed ocean, their relative velocity still does not exceed the speed of light. At such speeds, one cannot simply add the two velocities; instead one must use the velocity addition formula from the theory of special relativity. For details, see (for instance) http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html If you want to understand--*really* understand-- why the laws of physics are not the same in rotating reference frames, go to your local playground and try some simple physics experiments on the merry-go-round.```
 ```The answer is indeed incorrect as pointed out in the comment. The point which also should be made is that rotation is not described by special relativity. When we apply general realtivity, the apearance of centrifugal force does not contradict the relativity, as commenter suggest. That is actually meaning of the principle of equivalence. An observer in closed box cannot tell the difference between being accelerated or sitting in gravitaonal field. That is true locally (box is small - a point in fact). When you apply it to universe - it becomes very complicated and one has to consider cosmological models. The equations of general relativity are covariant (which is one way to make meaning of 'relative' more precise, but there is no real motion which would look like a gravity of a star. So answer is sort of yes and no and physicstt are still arguing bout weak and strong Principle of Equivalence. Either way, it is a good question and shows good physics intuition, which desreves better answers thanyou got so far. Pick up a good book.```
 ```Hi, I think your understanding of the Special Theory of Relativity is off by just a little. You say that it "concerns bodies moving at a constant speed." This isn't quite right. The Special Theory of Relativity concerns bodies moving at a constant velocity. Velocity takes into account both speed and direction of travel while speed says nothing about the direction. So to have constant velocity, both the speed and the direction of travel have to be constant. This is basically the same as saying that the object has zero acceleration. With angular motion the direction is always changing. This means that even if the speed is constant there is still acceleration. This also means that you cannot use the Special Theory, you must instead use the General Theory of Relativity. analogkid```
 ```"Since I posted my original question, I can see that my reference to Special Relativity could be confusing. The Google Answers systems does not allow me to actually edit my original question but please imagine that the following paragraph is inserted between the first and second paragraphs of my original question: In the General Theory of Relativity (which, unlike the Special Theory, also covers bodies not moving at a constant velocity) there is the Principle of Equivalence according to which, for example, the force felt by astronauts in an rocket in interstellar space which is held to be caused by the acceleration of the rocket in a certain direction, could with equal validity be accounted for by the gravitational field on the assumption that the rocket is stationary (or travelling at a constant velocity) and the stars are accelerating in the opposite direction."```